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Baby, explain why we are in this shape

31 March 2006

The Changing Role of the Embryo in Evolutionary Thought

It is hard now to recover a sense of how tenuous a hold Darwinism had in its early years. (Yes, I know about intelligent design, but I am talking about scientists.) The first major plank of his theory, that different species evolve from common ancestors, found favour rapidly.

But it was not until the 1930s that most biologists accepted the second; that natural selection accounts for evolutionary change. They were won over by a mathematically rigorous unification of evolutionary theory with the new science of population genetics, dubbed the "evolutionary synthesis".

It was a fair description, widely adopted after Julian Huxley coined it in 1942, but the synthesis was far from complete. Like the theory first outlined in On the Origin of Species , the synthesis set out to explain changes in living forms but not the forms themselves. It eventually justified this limitation with a history that portrayed a concern with form, with morphology, as not merely irrelevant to understanding evolution but anti-evolutionary.

Ron Amundson's aim is to explain how this history came to be told, to show that it is wrong and, he hopes, to prefigure a further synthesis of evolutionary theory and development that embraces change, form and changing form. His conviction that this is possible is reinforced by the advent of evolutionary developmental biology, or "evo-devo".

Practitioners see evo-devo as a new field, based on recent molecular-genetic discoveries about control of development. Only partly true, Amundson says. It is also a rediscovery of concerns shared by many biologists in the 19th century. Then, as now, it was logically true that an organism can assume a different form only by adopting a different path of development from embryo to adult. Every new creature descends from its parental line, but it also starts as a single cell. Understanding the significance of these facts for the shape it takes defines two aspects of the same question - how do organisms get to be the way they are.

The basic problem in bringing them together is that any general study of form invites consideration of the idea that there may be constraints on the way the organism can be beyond those imposed by natural selection. But suggesting that there are only so many options in the developmental repertoire sounded to defenders of Darwinism like a brand of idealism they abhorred. The idea that structuralism is the same as idealism was promoted by Ernst Mayr, evolutionary theorist and historian, around 1959. Amundson explains the reasoning behind it and Mayr's motives - largely to reinvent Darwin as the sole founding father of the evolutionary synthesis. And he delves deeply into the 19th century to show why what he calls the essentialism story was mistaken.

This is a philosopher's history, focusing on conceptual change, on metaphysical and epistemological commitments, and on what it mattered most to explain at particular times. As he re-evaluates past scientists and challenges earlier historical accounts, Amundson's credo is that "a scientific tradition forms itself, in part, by an active interpretation of its own history". He hopes to fashion just such an interpretation as a contribution to establishing evo-devo. As he says, it will be up to the scientists to produce a final synthesis of embryology and evolution, form and function, structure and adaptation. But if they manage it, here is some of the history they will need to tell a new story about the origin of theories.

Jon Turney is a visiting senior fellow in science and technology studies, University College London, and leads the MSc in creative non-fiction writing at Imperial College London.

The Changing Role of the Embryo in Evolutionary Thought: Roots of Evo-Devo